The prevailing control strategy in gait rehabilitation robotics is to enforce predefined motion patterns observed In human locomotion. Although this approach can restore steady human walking In well-defined environments, it does not equip the human-robot system with the functional dexterity that distinguishes legged mobility In natural unstructured environments. To overcome this fundamental limitation, the longterm objectives of the proposed work are to uncover the functional principles behind the biomechanical design and neuromuscular control of human legs In locomotion across gaits, and to transfer these principles to the design and control of powered leg prostheses and exoskeletons that restore or enhance the physical capabilities of Individuals who need locomotion assistance. The first specific aim In this application is to understand the biomechanics and neuromuscular control of the key leg behaviors of active balance recovery after large disturbances, continuous speed changes, combined walking and running, and gait transitions. The second specific aim Is to demonstrate and verify the advantages of the extracted principles to the control and dexterity of powered robotic legs. These aims will be addressed by deriving theoretical and computational models of neuromuscular control In human locomotion, and by developing a robotic leg testbed that replicates the actuation and dynamics of human and prosthetic legs and allows to rigorously characterize proposed leg designs and controls. The proposed work will help to reverse-engineer the sensorimotor control of human locomotion and has the potential to result In assistive technology that restores or enhances the physical capabilities of elderly people, patients, and amputees. Both directions can create a broad Impact on society, with particular benefits in public health care. The project directly relates to the NICHD research priority on medical rehabilitation for developing the scientific and technical knowledge needed to enhance health, productivity, Independence, and quality-of-life for people with disabilities.